1. Field of the Invention
The present invention relates to a semiconductor device such as a thin film transistor (TFT), a method of fabricating the same, a display device such as a liquid crystal display (LCD), and a method of fabricating the same.
2. Description of the Background Art
A thin film transistor (hereinafter referred to as a polycrystalline silicon TFT) employing a polycrystalline silicon film which is formed on a transparent insulating substrate as an active layer is recently being developed as each pixel driving element (pixel driving transistor) for an active matrix LCD.
The polycrystalline silicon TFT advantageously has larger mobility and higher drivability as compared with a thin film transistor employing an amorphous silicon film as an active layer. When such polycrystalline silicon TFTs are employed, therefore, an LCD of high performance LCD can be implemented while not only a pixel part (display part) but a peripheral driving circuit (driver part) can be integrally formed on the same substrate.
In such a polycrystalline silicon TFT, the polycrystalline silicon film for serving as an active layer can be formed by a method of directly depositing the polycrystalline silicon film on the substrate, a method of forming an amorphous silicon film on the substrate and thereafter polycrystallizing the same, or the like.
The method of directly depositing the polycrystalline silicon film on the substrate has a relatively simple step of depositing the film by CVD, for example, under a high temperature, for example.
On the other hand, the amorphous silicon film which is deposited on the substrate is thereafter polycrystallized by solid-phase crystallization in general. This solid-phase crystallization is adapted to polycrystallize the amorphous silicon film in a solid state by performing a heat treatment, for obtaining the polycrystalline silicon film.
An example of such solid-phase crystallization is now described with reference to FIGS. 31 and 32.
Step A (see FIG. 31): An amorphous silicon film is formed on an insulating substrate 51 of quartz glass, for example, by general low pressure CVD, and a heat treatment is performed in a nitrogen (N2) atmosphere at a temperature of about 900° C., thereby solid-phase growing the amorphous silicon film and forming a polycrystalline silicon film 52.
The polycrystalline silicon film 52 is worked into a prescribed shape by photolithography and dry etching by RIE, to be employed as an active layer of a thin film transistor.
A silicon oxide film for serving as a gate insulating film 53 is deposited on the polycrystalline silicon film 52 by low pressure CVD.
Step B (see FIG. 32): A polycrystalline silicon film is deposited on the gate insulating film 53 by low pressure CVD, an impurity is implanted into this polycrystalline silicon film, and a heat treatment is performed for activating the impurity.
Then, a silicon oxide film 54 is deposited on the polycrystalline silicon film by normal pressure CVD, and thereafter the polycrystalline silicon film and the silicon oxide film 54 are worked into prescribed shapes by photolithography and dry etching by RIE. The polycrystalline silicon film is employed as a gate electrode 55.
Then, an impurity is implanted into the polycrystalline silicon film 52 by self alignment through the gate electrode 55 and the silicon oxide film 54 serving as masks, for forming source/drain regions 56.
This method is called a high temperature process since high temperatures of about 900° C. are employed for the solid-phase crystallization and the impurity activation, and has such an advantage that the treatment time can be shortened when a substrate such as a quartz substrate, for example, having a high insulation property is employed.
However, such a substrate having a high insulation property is high-priced, while a relatively low-priced glass substrate unpreferably causes heat distortion. In recent years, therefore, development is generally made in a low temperature process which allows the employment of the glass substrate.
In particular, improvement of performance is indispensable in a TFT which is a driving device. Therefore, various attempts have been made in order to improve the quality of the material forming the TFT and the like through the low temperature process.
For example, a technique of forming a polycrystalline silicon thin film, for example, by excimer laser annealing with a starting material of an amorphous silicon film has been developed as a technique of improving the quality of an active layer material influencing the device characteristics.
However, the laser annealing disadvantageously requires a long time for the crystallization process, since a beam operation must be repeatedly performed. In case of employing only a laser beam as a heat source, the laser annealing requiring a long time must also be performed for activating an impurity region, for example, in addition to the polycrystallization process, and hence the total process time is increased to reduce the throughput of such a TFT device or an LCD device employing the TFT.